This invention relates to an improved air separation process wherein oxygen is produced at greater than atmospheric pressure.
Users of oxygen gas often require that the oxygen be delivered at a pressure greater than atmospheric pressure. In the past, this requirement has been met by compressing the oxygen gas to the desired pressure after the oxygen has been normally produced at low pressure in a cryogenic air separation plant. However, this method has significant disadvantages due to the explosive nature of highly compressed oxygen. Thus oxygen gas compression requires special care including special materials of construction, special lubrication techniques, and special compressor design to minimize possible metal to metal contact. It is common practice to place the oxygen gas compressor behind a concrete barrier to shield workmen and equipment should an explosion occur in the compressor. The hazards of oxygen gas compression increase as the pressure to which the oxygen must be compressed is increased.
In order to avoid the above mentioned difficulties, another method of producing oxygen at pressure has been devised. This method involves taking oxygen off the air separation column as a liquid, pumping the liquid to the desired pressure and then vaporizing the oxygen at that pressure. U.S. Pat. No. 2,784,372 to Wucherer et al describes such a method wherein argon is employed to vaporize the liquid oxygen.
Liquid oxygen pumping generally has not met with great commercial success to date primarily due to inefficiencies relates to distillation column performance. Because the oxygen is taken off as liquid, thermodynamic requirements dictate that liquid, sufficient to maintain an energy balance, i.e., equivalent in refrigeration value, be supplied to the column. In past practice, this liquid is supplied by condensing a sufficient portion of the incoming air stream to serve as the liquid makeup. Unfortunately, this results in downgraded column performance as that portion of the air stream which is liquefied bypasses some of the column separation.
Another method of producing oxygen gas at pressure involves recirculating nitrogen fluid to vaporize the liquid oxygen. This method is disadvantageous because nitrogen does not match the thermodynamic properties of oxygen resulting in process inefficiencies.
Oxygen at high pressure is increasing in demand especially as coal conversion and other synthetic fuel processes are increasingly employed. These synthetic fuel processes require oxygen gas at a pressure considerably above atmospheric. This increased pressure requirement makes oxygen gas compression a less desirable option. Therefore, a method by which oxygen gas can be produced at greater than atmospheric pressure and which overcomes the heretofore unavoidable degradation of column performance would be highly desirable.